a) Field of the Invention
The invention is directed to an arrangement for generating extreme ultraviolet radiation by means of an electrically operated gas discharge, containing a discharge chamber which has a discharge area for a gas discharge for forming a radiation-emitting plasma, a first disk-shaped electrode and a second disk-shaped electrode, at least one of which electrodes is rotatably mounted and has an edge area to be coated by a molten metal, an energy beam source for supplying a pre-ionization beam, and a discharge circuit connected to the electrodes for generating high-voltage pulses.
The invention is further directed to a method for generating extreme ultraviolet radiation by means of an electrically operated gas discharge for forming a radiation-emitting plasma from pre-ionized emitter material in which at least one rotatably mounted, disk-shaped electrode of a pair of electrodes provided for the gas discharge is coated in the edge area by a molten metal.
b) Description of the Related Art
Studies of a large number of electrode shapes for gas discharge sources such as, e.g., Z-pinch electrodes, hollow-cathode electrodes or plasma focus electrodes have shown that the life of electrodes constructed in these ways is insufficient for EUV projection lithography.
In contrast, rotary electrodes, as they are called, have turned out to be a very promising solution for appreciably increasing the life of gas discharge sources. One advantage is that these electrodes, which are disk-shaped in particular, can be cooled better. Another advantage consists in that inevitable electrode erosion can be prevented from shortening life by a constant renewal of the electrode surface.
A device previously known from WO 2005/025280 A2 uses rotating electrodes which are immersed in a vessel containing molten metal, e.g., tin, for regenerative application of a molten metal. The metal applied to the electrode surface is evaporated by laser radiation at the location where the two electrodes are closest together, whereupon the vapor is ignited by a gas discharge to form a plasma. The cooling of the electrodes is carried out by the metal baths.
The solution proposed in WO 2005/025280 has the following disadvantages: Because of the immersion process, the rotating speed of the electrodes is limited and is not sufficient for the required output specification of an EUV source. Owing to insufficient rotating speed, subsequent arrival of unconsumed electrode portions in the discharge area is too slow, which causes instabilities in the plasma generation. The rotating speed should be designed in such a way that the electrodes continue to rotate between two successive discharge pulses by an amount that is greater than the radius of the region of influence of the preceding discharge pulse on the electrode surface.
Because of the short dwell period of the electrodes in the molten metal, cooling the electrodes by means of the melt is insufficient for the required high output specifications. However, an additional cooling of the electrodes, for example, by a throughflow of water, would allow the temperature of the electrode surface to fall below the melting temperature of the metal applied by means of the molten baths during the prolonged pauses in the pulse operation provided for radiation generation which are common in exposure processes in semiconductor fabrication. This would result in a heavy, uncontrolled accumulation of the metal layer on the electrodes. Rapidly switching the additional cooling off and on would lead to a temperature gradient between the electrode surface and the interior of the electrode. Since this temperature gradient balances out when the additional cooling is switched off, an impermissibly high heating of the coolant can occur so that any gas bubbles that might possibly occur form a thermally insulating layer which prevents efficient cooling. Further, it is difficult to adjust the layer thickness of the applied material.